Due to the development of information and communication technology, there are various social problems due to a leakage of information. In other words, with the development of various wireless communication devices at present, various illegal eavesdropping devices and hidden cameras are abused and seriously infringe privacy and human rights of individuals.
Therefore, most countries are making efforts to respond this issue through legal regulations and policies. Individuals also try to prevent exposure to illegal eavesdropping and hidden camera by using various detection devices that can detect the eavesdropping and hidden camera at the individual level.
As a result of the above efforts, a technique has been proposed in Korean Patent Publication No. 10-2006-0021803, entitled apparatus detecting and preventing a portable wire/wireless hidden tapping device, a laser hidden tapping device, and a hidden camera. In the publication, a precise position transmitted to/from a wireless eavesdropper can be searched easily without a user operation by measuring the field intensity of radio frequency transmitted from a wireless eavesdropping device and comparing whether the measured electric field intensity is higher than reference electric field intensity.
In addition, Korean Patent Publication No. 10-2008-0048380, entitled propagation detection method for an illegal wireless device, a propagation detection circuit, and a mobile phone equipped with the propagation detection circuit, proposed a technique to detect a radio signal transmitted from a wireless eavesdropping device and a hidden camera. The device receiving and processing the signal is integrated in a form of a micro device and a chip, modularized into a single chipset, and incorporated in a mobile phone to independently perform its function.
In addition to the above described techniques, conventional art proposes a method for preventing eavesdropping and recording by generating white noise to prevent eavesdropping or recording by an eavesdropping device.
When a sound A and a sound B coexist and the sound wave B has larger amplitude than the sound wave A, the sound wave A is not heard to a human ear. In this case the sound wave A is masked by the sound wave B, and this phenomenon is called a masking effect.
On the other hand, electrical or mechanical devices such as a microphone for receiving sound waves collect only sound waves. When an unspecified noise is mixed up, the original sound waves are masked by the noise and thus sound reception ability is limited. An eavesdropping device should be located within a sound field, which is a place where sound waves exists, to collect conversations or conversations.
A method of preventing eavesdropping using white noise applies a masking effect and interferes with target sound waves received by a microphone of the eavesdropping device by providing unnecessary sound wave (noise). As shown in FIG. 1, the method of preventing eavesdropping using white noise generates noise having an audible frequency band to cause a masking effect. The noise having the audible frequency band prevents the target sound waves, i.e., the original conversation waves, from being tapped, wiretapped or recorded.
FIG. 2 shows how to generate the white noise. A collector of the transistor A is laid open, and a base and an emitter are combined to generate the noise of the transistor (A). The noise, then, is passed to the BPF (B: Band Pass Filter). The BPF (2) filters only the noise within 20 Hz to 20 kHz, which is a desired audible frequency band. The filtered noise is amplified by an AMP (3: Audio Amplifier) and output through the speaker (4).
As mentioned above, the conventional method for preventing eavesdropping using white noise generates a white noise in an audible frequency band and works only when white noise has intensity larger than intensity of a target sound wave, e.g., a human voice. This is disadvantageous to use.
The conventional method is disadvantageous in that: (i) the white noise is generated during a conversation between subject parties, and thus the subject parties may easily notice that their conversation is tapped or wiretapped; (ii) the noise generator should be closely positioned to a target area where the conversation is exchanged, (iii) it is difficult to use when the target area is wide, and (iv) the noise is too loud, making the conversation or the communication difficult.